This is a national phase application based on International Application PCT/JP98/05965 filed Dec. 25, 1998. International Application PCT/JP98/05965 was not published under PCT Article 21(2) in the English Language.
This invention relates to a continuous annealing furnace, to a roller bearing, to an annealing method and to a method for manufacturing an inner ring and an outer ring for a deep groove ball bearing. In particular, this invention relates to a technique which is useful for the manufacturing machine parts mainly concerning a roller bearing to be employed in various portions of a two-wheeler, a motor car, an agricultural machinery, a construction machine, etc.
(1) FIG. 1 illustrates a method of manufacturing an inner ring and an outer ring for a deep groove ball bearing which involves a turning work after a heat treatment, and which is generally employed in view of saving the manufacturing cost.
As for the raw round bar, a steel round bar which has been rolled is generally employed as it is (S1). Then, a hot forging is performed generally using a multi-stage former thereby to produce a rough ring to be subsequently finished into an inner ring or an outer ring (S2). After this hot forging, by means of softening, the hardness of the rough ring is lowered or the microstructure of the rough ring is improved for the convenience of the following steps (S3). Then, a cold rolling work (hereinafter, referred to as CRF working) is performed (S4). By the way, this CRF working is performed, as schematically shown in FIG. 5, for thinning the cross-section of the rough ring 23 and at the same time, for enlarging the diameter of the rough ring 23 by rolling the rough ring 23 interposed between a molding roll 21 and a mandrel 22 under a load of xe2x80x9cWxe2x80x9d on the rough ring 23.
FIG. 3 illustrates a forging process wherein the aforementioned CRF working is not employed. Namely, in order to fabricate forged rings 1 and 2, a ring 3 and a fraction 4 are discarded as a scrap. On the other hand, when the CRF working is employed, small forged rings 5 and 6 are produced as shown in FIG. 4(D) in a hot forging step, and then, these forged rings 5 and 6 are diametrally enlarged to form CRF rings 7 and 8 as shown in FIG. 4(E). Therefore, the ring 3 shown in FIG. 3(D) is no more required to be produced, and the fraction 4 shown in FIG. 3(D) can be also made smaller as represented by a fraction 12 as shown in FIG. 4(D). Furthermore, a groove 7a or 8a may be formed on the raceway surface of the CRF ring 7 or 8, respectively, at the occasion of obtaining a diametrally enlarged CRF ring 7 or 8, thereby minimizing as much as possible the magnitude of turning, thus resulting in an excellent saving of material.
Next, a sizing of the ring is performed (S5) as shown in FIG. 1. Subsequently, the entire surface of the CRF ring is subjected to a turning work (S6) thereby to remove an oxide layer or a decarbonized layer that has been formed during the forging or annealing, or thereby to form the CRF ring into a deep groove ball bearing after a heat treatment or grinding. Depending on certain circumstances, a flash that may happen to be formed during the hot forging may be removed by means of grinding before the aforementioned turning work. Then, the ring is subjected to a quenching and an annealing thereby to obtain a suitable hardness of the ring required for the bearing (S7). Next, the raceway surface and the fitting surface of the ring is subjected to a grinding work thereby to manufacture an inner ring and an outer ring (S8).
Japanese Patent Publication H6-83872 discloses a method wherein a rough ring is subjected to a turning work thereby making the ring fixed in configuration and in weight prior to the CRF working, and then, the resultant ring is subjected to a high precision CRF working in which the ring is completely finished to form a raceway surface groove as well as a sealing groove which are desired as a deep groove ball bearing, thereby making it possible to omit the subsequent turning work, and to save the manufacturing cost.
By the way, this CRF working is a process, as mentioned above, for enlarging the diameter of the rough ring (such as a forging ring or a turning ring) thereby to make the rough ring into a CRF ring. Therefore, although it is possible to adjust the thickness and the magnitude of enlarging the diameter of the ring, it is impossible to control the dimension in the lateral direction of the ring, thus allowing a flash to be generated in the lateral direction of the ring.
In the case of the method disclosed in Japanese Patent Publication H6-83872 where a high precision CRF working is performed, a flash extending in the lateral direction is caused to be generated, or fine cracks due to the cold working are also caused to be generated at the edge portion of a sealing groove forming a complicated configuration or of a race groove. Therefore, not only a full-face turning work for adjusting the dimension or for making the volume constant is required to be performed prior to the CRF working, but also a finishing turning work for removing the fine cracks is required to be performed after the CRF working. Further, when a complicated configuration is to be obtained through a high-precision CRF working, the working time would be prolonged and the cost for the CRF working would be increased.
On the other hand, in the method where a turning work is not performed prior to the CRF working as shown in FIG. 1, a groove (the groove 7a or 8a in FIG. 4) is usually formed on the raceway surface at the occasion of enlarging the diameter of the ring by making use of the CRF working so as to minimize as much as possible the magnitude of turning. However, the magnitude of turning is excessively minimized, the decarbonized layer that has been formed during the hot forging or softening may be left remained after the turning work, thus raising a problem in terms of the function of the bearing. As a result, a full-face turning work is also required to be performed on the ring after the CRF working, thus leaving a room for further improvement regarding the reduction of manufacturing cost through a simplification of manufacturing steps.
Since the bearing is subject to a repeated load at a small contacting surface thereof, i.e. between the raceway surface thereof and a rolling element, the bearing is generally formed of a hard steel which is capable of withstanding this repeated load (stress) and has a uniform structure and an excellent abrasion resistance. Therefore, a high carbon chromium bearing steel (SUJ2) which is defined in the JISG 4805 is typically employed for the bearing. This bearing steel is featured in that, in view of ensuring a high hardness, the content of carbon in this raw material is as high as about 1%.
Therefore, when this bearing steel is heated at a high temperature in the air atmosphere in the manufacturing steps thereof, the surface of the bearing steel is decarbonized, thus failing to obtain a predetermined hardness if this heat-treated bearing steel is to be employed as it is. This means that if a decarbonized layer is left remained on the rolling contact surface of the bearing after the manufacture thereof, the capabilities of the bearing such as the life and abrasion resistance thereof may be deteriorated.
In the ordinary manufacturing method of the bearing ring, a method wherein a steel bar obtained from rolling is directly formed into a rectangular ring by means of a hot forging, and then, the resultant ring is subjected to a finishing work by means of turning work, or a method wherein a tubular material is subjected to a finishing work by means of turning work has been employed. There are also known various working methods for forming a ring. In any of these working methods however, a spheroidizing annealing is performed prior to the turning work in order to facilitate the workability of raw material.
The decarburization of the bearing steel is caused to occur at first in the processing of raw material among the steps of manufacturing the bearing ring. Namely, a raw material for the bearing is formed into a billet by making use of a blooming mill and then, worked into a bar, a tube or a wire by means of a hot rolling, thus preparing a raw material for the manufacture of a bearing. Since the billet in the blooming mill, or the bar or tube in the hot rolling is subjected to a heating at a temperature of as high as over 1,150xc2x0 C. or around 1,200xc2x0 C. in air atmosphere, a decarbonized layer of as thick as several micrometers to several hundreds micrometers may be formed on the surface of the bar, tube, etc., i.e. the generation of decarburization has been an unavoidable problem. Moreover, when a ring is to be formed using such a raw material by means of hot forging, the ring is again exposed to a high temperature (about 1,150xc2x0 C.) in air atmosphere, the decarbonized layer or the resultant ring becomes deeper as it is combined with the decarbonized layer formed initially in the stage of raw material. Additionally, the concentration of carbon in the surface layer of the ring is also caused to decrease further.
This decarburization may be recognized also in the spheroidizing annealing that will be performed prior to the turning work or the finishing work of grinding. The spheroidizing annealing of bearing steel has been conventionally performed prior to the turning work in order to facilitate the workability of raw material. For example, a typical method of the spheroidizing annealing is set forth in the publication: xe2x80x9cHeat-treatmentxe2x80x9d, Vol. 18, No. 1 pp. 22. Namely, the ordinary method thereof comprises the steps of heating a bearing steel up to a point (780 to 810xc2x0 C.) immediately over the Al transformation temperature thereof, and annealing the bearing steel at a temperature (730 to 750xc2x0 C.) in the vicinity of the Al transformation temperature thereof. Although the temperature employed in this heat treatment is lower than the temperatures to be employed in the hot forging or the carburizing treatment, since the bearing steel is exposed to a heating for a period of as long as more than 12 hours, the carbonization thereof would be further promoted if the treatment atmosphere employed is formed of an oxidizing atmosphere such as air atmosphere.
With a view to solve this problem, a method of removing the decarburization by means of a turning or grinding work has been applied to an ordinary raw material. Further, for the purpose of completely removing a decarbonized layer or obtaining a precise configuration of raceway surface after a spheroidization annealing, a turning or grinding work has been applied also to the ring that has been finish-worked or turn-worked thereby to remove the decarbonized layer formed thereon. However, these methods invite a big increase in manufacturing cost due to an increase in manufacturing step and to a lowering of yield.
In recent years, in view of improving the productivity and saving the manufacturing cost, the magnitude of cutting in the finishing turning or grinding step is desired. Further, since it has become possible to precisely form the ring by means of cold rolling to be performed after annealing, the turning or grinding work itself can be sometimes dispensed with. Accordingly, in order to reduce the turning or grinding step (a reduction of manufacturing steps), it is now desired that any decarbonized layer is no more left after the spheroidization annealing step.
Therefore, in view of minimizing the decarbonized layer, a method of applying a carburizing treatment to a raw material after rolling (such as bar, tube or wire) thereby to minimize the decarbonized layer has been variously proposed and actually practiced. For example, in the case of a coil element for wire rod, an atmospheric annealing is employed at the occasion of the spheroidization annealing step. However, in the case of the wire rod, once a carburization has been generated by means of the atmospheric annealing, the surface layer is hardened so that a working flaw may be generated at the occasion of drawing.
Therefore, although the depth of decarburization is required to be precisely controlled, it has been difficult according to the prior method to control the depth of decarburization. The spheroidizing annealing generally comprises the steps of heating a work piece up to a point (about 800xc2x0 C.) immediately over the Al transformation temperature thereof, and gradually cooling the work piece down to a temperature lower than the Al transformation temperature (about 700xc2x0 C.). Therefore, if a carburizing gas atmosphere is employed at a relatively low temperature, a sooting is caused to generate (the precipitation of soot). If the soot is adhered onto the work piece, an additional step for removing the soot is required to be included in the subsequent steps, and therefore, the cost for maintenance of the furnace is additionally required.
As a method of minimizing the decarburization in the spheroidizing annealing step in the case of a bearing steel of high carbon, a method of recarburizing the decarbonized layer that has been generated in the rolling step in the manufacture of a raw material has been conventionally proposed (for example, Japanese Patent Unexamined Publication H7-37645; Japanese Patent Publication H7-30438; Japanese Patent Unexamined Publication H2-54717; and Japanese Patent Unexamined Publication H5-148611). According to Japanese Patent Unexamined Publication H5-148611 and Japanese Patent Publication H7-30438, for the purpose of preventing and minimizing the decarburization, the recarburization of the decarbonized layer is performed in a carburizing atmosphere. Further, according to the methods disclosed in these publications, for the purpose of performing the spheroidizing annealing, a work piece is once heated up to the Al transformation temperature or more, but the control of carburization is performed mainly at a lower temperature zone with a view that the carburization (recarburization) of the work piece can be preferably performed at the temperature zone which is not more than the Al transformation temperature (for example, 710xc2x0 C. or less).
However, even in this recarburization method which is to be performed at a low temperature which is not more than around the Al transformation temperature, the gas atmosphere to be employed in the recarburization is required to be formed of a carburizing gas, and at the same time, the work piece is required to be gradually cooled while controlling the carburizing gas. In the case of this method also, since a carburizing gas atmosphere is employed at a relatively low temperature as in the case of the atmospheric annealing of a wire rod, soot may be caused to precipitate thus generating a sooting, and therefore, it requires a precise control of the carburization.
Further, in the case of the method shown in Japanese Patent Unexamined Publication H5-148611, even though the carburization (recarburization) of the work piece is performed also at a temperature which is not more than the Al temperature, a work piece is rendered to pass through a carburizing gas atmosphere even in a homogenous heating zone (carburizing zone) of not less than the Al temperature. Accordingly, even if the control of atmosphere (carburizing gas atmosphere) is performed at a temperature of not more than the Al temperature, the level (high or low) of setting the atmosphere in the homogenous heating zone gives a big influence to the concentration of carbon in the surface layer of the work piece after the annealing, thus leaving a room for improvement with regard to the control of atmosphere in the homogenous heating zone as well as at the temperature of not more than the Al temperature.
According to Japanese Patent Unexamined Publication H7-37645, after a carburization is performed, the resultant work is required to be re-heated in a direct type annealing furnace filled with an oxidizing atmosphere so as to remove the carburization layer, thereby adjusting the concentration of carbon in the surface layer, thus resulting in an increase in manufacturing cost. Further, according to Japanese Patent Unexamined Publication H2-54717, a continuous process is set forth. However, this continuous process is simply formed of a combination of a carburization treatment and a spheroidizing annealing treatment, thus simply prolonging the treatment time more than required.
Since the recarburization treatment of a raw material is performed by taking the processing of the raw material in the subsequent steps into consideration, the decarburization can be minimized. However, this recarburization treatment should be performed so as not to render the surface or the work piece to become excessively carbonized thereby to give a bad influence to the subsequent steps. Accordingly, some of the prior methods require a separate step of decarburization treatment after the recarbonization treatment, thus making the method more complicated, while other methods require a carbonization treatment prior to the annealing treatment, thus prolonging the entire processing time.
In order to perform a full recarbonization treatment in a spheroidizing annealing step at a low cost, the control of carbonization atmosphere at low temperatures or a follow-up control of carbonization atmosphere in relative to changes in temperature at low temperatures is required. Additionally, it is also required that a treatment method or a treatment furnace which is capable of satisfactorily performing a recarbonization treatment even if a large number of works are piled up in bulk and contacted with each other.
This invention has been accomplished under the circumstances as explained above and therefore, an object of the present invention is to provide a continuous annealing furnace which is capable of performing a recarburization of decarbonized layer formed in a hot forging in a stable manner and at a low cost, wherein a continuous annealing is performed by making use of two or more treatment chambers and an intermediate chamber interposed between the treatment chambers and partitioned by means of an openable door from the treatment chambers, the two or more treatment chambers and the intermediate chamber being sequentially connected with each other, and each treatment chamber being filled with a gas atmosphere differing in carburizing property from that of the other chambers.
Another object of this invention is to provide an annealing method which is featured in that the recarburization is performed by making use of the aforementioned continuous annealing furnace.
Another object of this invention is to provide a roller bearing which is long in life and excellent in workability.
Further object of this invention is to provide, through a combination of optimum working steps, a method for manufacturing an inner ring and an outer ring for a deep groove ball bearing, which are excellent in capabilities as a bearing, the method enabling the manufacturing cost to become as minimum as possible.
(1) This invention provides a continuous annealing furnace for enabling a continuous spheroidizing annealing of a high carbon bearing steel to be performed, the continuous annealing furnace comprising; two or more treatment chambers sequentially connected with each other and respectively filled with a gas atmosphere differing in carburizing property from that of the other chambers; an intermediate chamber interposed between the treatment chambers and partitioned by means of an openable door from the treatment chambers; and transferring means for passing a work piece to be treated through the two or more treatment chambers and the intermediate chamber.
(2) This invention also provides a roller bearing which is formed of a high carbon bearing steel treated as mentioned in the above item (1), and whose finished raceway surface exhibits a maximum carburization ratio of 0.1% (more preferably 5%) to 30% and a carburization depth of 0.1 mm to 0.5 mm.
(3) This invention also provides a method for manufacturing an inner ring and an outer ring for a deep groove ball bearing wherein a grinding work is performed after a heat treatment, the method being applicable to the softening furnace and comprising the steps of; hot-forging a raw round bar thereby to form a rough ring to be worked into an inner ring or an outer ring; softening the rough ring so as to spheroidize carbides contained in the rough ring for a convenience of subsequent cold working or turning working; cold-rolling the rough ring while holding the inner circumferential surface and outer circumferential surface of the rough ring between a mandrel and a roller, thereby enlarging the diameter of the rough ring to form the configuration of the inner ring or the outer ring; turning the diametrally enlarged ring thereby finishing it into a configuration required for a deep groove ball bearing; performing a hardening and a tempering of the ring thereby to obtain a hardness required for a bearing; and grinding the raceway surface and fitting surface of the ring.
(4) This invention also provides a method for performing a continuous spheroidizing annealing of a high carbon bearing steel, which is characterized in that the annealing is performed by passing a work piece to be treated through an annealing furnace comprising two or more treatment chambers sequentially connected with each other and respectively filled with a gas atmosphere differing in carburizing property from that of the other chambers, an intermediate chamber interposed between the treatment chambers and partitioned by means of an openable door from the treatment chambers.